This invention relates generally to a method for heat treating Ti/Al-base alloys to improve their resistance to embrittlement when exposed to elevated temperatures or creep conditions. More particularly, the method comprises aging Ti/Al-base alloys which comprise a mixture of .alpha. (Ti solid solution) and .alpha.2 (Ti.sub.3 Al) phases over a plurality of successively lower elevated temperatures that are each lower than the .alpha./.alpha.2 solvus temperature in order to control the particle size and distribution of the .alpha.2 phase.
Embrittlement after thermal exposure in titanium alloys is a well-known phenomena. For example, it known that binary Ti/Al alloys are embrittled by the occurrence of a second phase, see Crossley et al., Embrittlement of Ti--Al Alloys in the 6 to 10 Pct Al Range, Journal of Metals, Transactions AIME, pp. 43-46 (January 1957). This embrittlement has also been reported to result in tensile deformation that occurs in sharp bands within Ti-8 Al (in weight percent) alloys due to the shearing of fine Ti.sub.3 Al particles; see Boyd et al., Slip in Titanium Aluminum Alloys Containing Small Ti.sub.3 Al Precipitates, pp. 1071-1083, an article in a compendium edited by Jaffee et al., Titanium Science and Technology, Cambridge, Mass. (May 2-5, 1972). This type of embrittlement phenomena is significant, because it occurs at elevated temperatures and when the alloy is under a load, conditions for which it is generally desirable to utilize Ti/Al-base alloys. Thus, this phenomena serves as a significant limitation on the use if these alloys.
It has also been reported that the tensile ductility of Ti--Al solid solution alloys can be improved somewhat by long time isothermal treatments at high temperatures to form large diameter Ti.sub.3 Al precipitates. The mechanism associated with this improvement was reported to be a transition from dislocation cutting to dislocation by-pass related to increases in the particle size and inter-particle spacing which resulted from the isothermal heat treatment, see Lutjering et al., Mechanical Properties of Age-Hardened Titanium-Aluminum Alloys, Acta Metallurgica Vol. 18, pp. 785-795, (1970). However, this work only reported on the results due to isothermal aging, and further reported that small precipitates remained in these isothermally heat treated alloys upon cooling to room temperature. Also, this reference did not discuss the behavior of these alloys under creep conditions, i.e., under load at an elevated temperature.